This graph shows part of a single scan of the sky centered on SNR 1181.
The X axis is the Right Ascension coordinate, which indicates
where the telescope was pointing. The Y axis shows the
total system temperature measured by each of two receivers, one with a
center frequency of 8.35 GHz (3.6 centimeter wavelength, red) and
14.35 GHz (2.1 centimeter wavelength, green).
Notice that there is only a very small variation in the system temperature measured along the scan. This is because astronomical object contribute a very small amount to the total detected power.
|Receiver||Receiver Noise adds temperature to the measurments (lower noise recievers are better). For the GPA survey, both receivers have a temperature of about 45 Kelvins. This temperature is independent of the direction the telescope points and is easy to calibrate and remove.|
|Sky||Sky brightness adds temperature due to water vapor in the air. Water vapor (clouds) both absorbs radio waves and warm. In perfect weather, the sky temperature is about 6 degrees. In bad weather the sky can add over 100 degrees. Also clouds are clumpy and have different temperatures in different directions, which makes them difficult to calibrate. Useful survey data can only be obtained at these wavelengths when the weather is very clear.|
|Cosmic||A significant component to all measurements of the brightness of radio sources is the Cosmic Background radiation, which is the remaining emission from the Big Bang beginning of the Universe. This Cosmic Background Radiation is very similar in all directions of the sky and adds 2.7 Kelvins to all measurements.|
The sky brightness and receiver temperature components of the
measurements are removed by fitting a curve to the ends of the measurements
of brightness and subtracting this fit.
The source SNR 1181 is clearly visible in these plots after removing the other effects. Notice that SNR 1181 is brighter at 8.35 GHz than 14.35 GHz. Also notice the noise in the scan is greater in the 14.35 GHz data. This is because the sky contribution to the 14.35 GHz data is much greater.
For the GPA survey, the brightness temperatures were converted to Janskies by observations of a number of compact bright radio sources with known Flux Densities. These sources are listed below:
|Cygnus A||Radio Galaxy|
|NGC 7027||Planetary Nebula|
|3C 286||Distant Quasar|
After calibration, the measurements are averages and placed on a grid and
a map of the region is produced, with units of Janskies.
The final GPA maps are produced by averaging four independent images of the same regions of the sky. This process reduces the noise in the observations and provides a way of removing isolated errors in the images, such as bad weather.